2. Field of the Invention
This invention relates generally to shaft seals and more particularly relates to a debris-excluding shaft seal with an extended life expectancy achieved through minimization of frictional contact between rotating sealing surfaces and through entrapment of abrasive contaminants within a porous filter element.
2. Description of Prior Developments
Prior debris-excluding seals have traditionally experienced various operational problems including the generation of excessive heat via frictional contact between rotating seal elements. This frictional heat is primarily caused by the relatively high contact forces extisting between the rotating seal elements. While these high contact forces were believed necessary to effect a reliable seal, they have been shown to cause rapid wear between the rotating sealing surfaces and have significantly reduced seal life.
One approach to overcome this heating problem is to avoid contact between the rotating elements by providing a small clearance therebetween. However, this approach presents another problem wherein debris passes through, or is entrapped within the clearance space. This condition results in extremely rapid abrasive wear of the sealing elements as well as wear of the rotating shaft.
It has generally been assumed that by providing a plurality of engagement points between rotating seal elements so as to form a plurality or series of sealing surfaces, a more effective barrier to debris would result. Thus, known debris excluding seals generally provide several points of contact between the rotating seal elements to form a series of sealing surfaces. However, each contact point acts as a source of frictional heat, which as noted above, adversely affects seal life.
A particularly difficult sealing problem arises in those applications where dirt excluding seals must accommodate significant shaft runout and/or shaft-to-bore misalignment. Prior seals have not proven effective in these cases and have not been well accepted by industry.
Another inconvenience associated with existing dirt excluding seals is the difficulty of properly and accurately aligning the stationary seal element with respect to the rotating seal element during installation of the seal assembly about a shaft.
In those instances where a seal is provided with hydrodynamic features, an additional problem often arises. While the hydrodynamic pumping effect serves to prevent lubricant from passing beyond the seal lip, a vacuum is generated which tends to draw or suck air beneath the lip from the surrounding environment on the dry or air side of the seal. If the air contains any contaminants such as dust particles, the contaminants are also drawn under the seal lip. This not only accelerates lip wear and reduces seal life but it also results in contaminanted lubricant and/or wear of internal moving parts.
When a seal produces a hydrodynamic action a surprisingly large pressure differential is developed across the seal lip. As noted, a vacuum effect is created on the air or dry side of the seal while ambient pressure is present within the wet or inner side of the seal. This condition can result in large contact forces between the lip and shaft if an air tight seal is formed therebetweem. This contact force can be reduced by allowing a small amount of air to be drawn past the seal lip so as to minimize the pressure differential. However, by drawing air beneath the seal lip, the problem of contamination arises.
Moreover, large contact forces such as described above generate excessive frictional heat. In some instances this heat between the shaft and lip becomes so great that the lubricant which flows against and beneath the lip breaks down and becomes corbonized. This carbonized lubricant then bonds to the lip and/or shaft and forms a deposit of build-up of hardened carbon. This build -up then prevents continuous contact between the lip and shaft and results in seal leakage.
Accordingly, a need exists for a unitized dirt-excluding shaft seal which minmizes the generation of frictional heat by reducing the contact force between rotating sealing surfaces thereby extending seal life and protecting shaft finish.
A need also exists for a seal which prevents contaminants such as dust particles from passing beneath a seal lip, particularly in those cases where the seal is generating a hydrodynamic effect, yet which avoids large contact forces between the seal lip and the shaft.